1. Field of the Invention
The present invention relates to optics and, more particularly, to high numerical aperture optical elements particularly suitable for microscope objectives or for focusing or collimating light from coherent light sources.
2. Description Relative to the Prior Art
In a book entitled "Photomicrography" by Roger P. Loveland, published by John Wiley & Sons, Inc., N.Y. (1970), p. 532-533, the statement is made that: "In 1906, Siedentopf of Zeiss developed the cardioid dark field reflecting condenser. The cardioid is a heart-shaped curve with a familiar cusp. The surface of revolution on the axis of symmetry has some interesting optical properties. A spherical surface can be placed so that all rays originally parallel to the axis will be focused through the point of the cusp. Moreover, the resulting catoptric lens is aplanatic, anastigmatic, and of course, completely achromatic."
In 1932, D. D. Maksutov in U.S.S.R. Patent No. 40859, entitled Description of High Aperture Objective for Microscopes, described a solid version of this two surface reflecting system where the entrance and exit surfaces were defined normal to the rays so that no angular deviation occurred at either surface and the index of refraction of the medium was arbitrary. When used in a system having finite conjugates the asphere required is not exactly a cardioid, and, thus, the system is not exactly aplanatic.
The optical performance of the above-mentioned systems is good, but, because both systems are reflective systems, they both suffer from energy losses due to obscuration and are not suitable for applications requiring the combination of relatively high apertures, short focal lengths, and long working distances.
A refractive system meeting the foregoing criteria can be designed with spherical surfaces but requires many elements. However, to achieve a single element system, it is possible, for example, to use non-spherical so-called Cartesian surfaces to define a single element plano-convex lens that takes a collimated beam to a point focus. The parallel rays enter perpendicular to the plano surface and are undeviated until they exit through the convex surface, which is defined as a surface of revolution with a hyperbolic cross section, whose conic constant is the negative index of refraction squared and whose vertex radius is the desired back focus times the index of refraction minus one.
Although such a lens element is corrected for spherical aberration, it is not aplanatic, and thus, can not be used for very high numerical aperture applications.
Similarly, using conventional aspheric lens design programs, one can define such a lens with one or two aspheric surfaces. Such a lens will be corrected for spherical aberrations and may be corrected for coma. These lenses are commonly employed in CD players and recorders. However, it is difficult to design and manufacture such a lens with a numerical aperture beyond 0.55. In addition, these lenses tend to have a small working distance.